[Straight Skeleton] Fix #143 - ST_StraightSkeleton returns degenerate line segments

@robe2 I've added an optional 'tolerance' (1e-8) parameter to exclude 'degenerate' linestring.

Tested with postgis31 and this branch, it's ok for me:

test=# WITH bt(box) AS (
SELECT st_makebox2d(st_point(1296000,202400),st_point(1297000,212400))
)
SELECT d.path[1], St_astext(d.geom) , ST_Length(d.geom)
FROM bt, st_dump(
st_straightskeleton(st_difference(st_buffer(box,2000,2),box))
) as d
WHERE st_startpoint(d.geom) = st_endpoint(d.geom);
 path | st_astext | st_length 
------+-----------+-----------
(0 ligne)